An internal combustion engine operates in a certain range of speed and power. Inside this range, there usually exists a smaller regime where the engine achieves optimal performance. On the other hand, driving conditions vary enormously, not only in wheel speed but also in driving torque at the drive wheels. A combination of a given speed and torque defines a power state. Matching the power state of the engine with that of the drive wheels is one of the primary functions for a transmission.
In recent years, the development of hybrid technology provides new avenues for achieving improved matching in power state between the internal combustion engine and the drive wheels. Among various power-train architecture designs, the most representative is the electro-mechanical continuous variable transmission, known as Toyota Hybrid System, or THS. THS uses the power split principle, splitting the input power into two paths of different type. Part of the input power passes through a mechanical power path which is comprised of gears and shafts; the rest passes through an electric power path which contains electric machines. The device that splits the power is a simple planetary gear system. THS has only one power splitting mode and provides a single output to input speed ratio node point SR where all power passes through the mechanical power path. When the transmission operates at a speed ratio higher than the speed ratio node point, internal power circulation occurs. One of the power paths sees more power than the power transmitted through the transmission, which reduces power transmission efficiency. This, to a large extent, constrains the effective operating speed ratio of the transmission. For high power vehicle applications, the power ratings for the electric machines have to be increased significantly.